1. Field of the Invention
This invention relates broadly to radar systems. More particularly, this invention relates to radar systems for helicopters which utilize modified helicopter rotor blades.
2. State of the Art
There is a need for surveillance radar equipment optimized for helicopter use. Such surveillance radar should be able to detect moving and stationary targets anywhere (360.degree.) around the helicopter, obtain target signatures using range profiles, and track while scanning. Potential uses include landing area identification, bomb damage assessment, and other tactical applications. When the need for such radar systems is coupled with the requirements for size and weight limitations for a helicopter, and the low observability required, the design of an acceptable radar system becomes very difficult. In addition, the location and size of the radar system impacts the dynamics of helicopter flight and plays a significant role in the ability of the helicopter to safely and effectively perform its mission. Referring to prior art FIG. 1, surveillance typically requires the ability to map a terrain over 360.degree. in a range (R) of 5 to 10 km, with a range accuracy (.DELTA.R) of 1 meter, and an angular resolution (.DELTA..theta.) of one degree. The range accuracy may be achieved by a waveform design using either a short pulse or a pulse compression technique, each having a bandwidth, .DELTA.f=1/T, where T is the equivalent pulse width corresponding to a 1 meter range. Since T=.DELTA.R/2c, where c is the speed of light, T=1.6.times.10.sup.-9, and .DELTA.f=600 MHz. The beam width (.DELTA..theta.) of a linear radiating structure of length D (i.e., the radar reflector) is given as .DELTA..theta.=.lambda./D radians, or 57.3 .lambda./D degrees where .lambda. is the wavelength of the radiation and is given as: .lambda.=c/f. For .DELTA..theta.=1 degree, and a frequency of 35 GHz; i.e., .lambda.=8 mm, D=0.5 meters.
The location of a 0.5 meter 360.degree. scanning reflector on a helicopter poses a problem for all but the largest helicopters. An acceptable, though imperfect, solution for a large helicopter 10 is to mount a rotating reflector 12 in a radome 14 on the top mast 16 of the helicopter, overlooking the blades 18 and body 20 of the helicopter 10. However, this type of installation can have a negative impact on helicopter dynamics by adding intolerable drag and excessive weight to the helicopter. In addition, top mast mounted radar reflectors provide a large radar cross section which dominates the backscatter from the helicopter and enables the helicopter to be more easily detected by others.
In smaller helicopters, the radar systems are often provided in the nose of the helicopter. However, such systems are forward looking only and, therefore, provide only limited terrain coverage. Moreover, forward looking radar systems are generally unable to image objects located above the helicopter, as may be particularly needed in military applications or for detection of overhead cables where the helicopter is flying very low.